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Blending turbulence

We have made significant enhancements to the representation of physical processes and parametrizations in our regional forecast models. One of the main changes is the blended turbulence scheme which improves the way the mixing generated by swirling gusts of wind - or turbulent eddies - is represented in the model. Professor Simon Vosper, Head of Atmospheric Processes and Parametrizations, describes the benefits these changes will bring.

We are continually developing our weather and climate models to improve their accuracy and provide new capability. As featured in a previous edition of Barometer, we made a major change to our global prediction systems last year by implementing the ENDGame dynamical core in global versions of the Met Office Unified Model.

ENDGame provides a more accurate solution to the equations of motion for the atmosphere and results in more detailed and reliable forecasts of weather features such as cyclones, fronts and jet stream winds. At the same time as implementing ENDGame we also introduced major improvements to the physical parametrizations in the global model - these represent the complex small-scale phenomena that cannot be explicitly resolved by the model, such as turbulence, cloud processes, solar heating and the effects of hills and small mountains.

Improving regional forecast models

Following upgrading the global model, we introduced ENDGame dynamics into regional versions of the model. These configurations run at much higher resolution, providing detail on the scale of only a few kilometres, but over limited areas. The highest resolution version is the UKV model, which has a grid spacing of 1.5 km over the UK. This model is used to provide a detailed picture of our weather across the UK, and is able to forecast hazardous weather such as individual convective storms and account for how these are influenced by local features such hills and valleys.

Another important effect of hills is the generation of lee waves - atmospheric wave patterns which can extend for hundreds of kilometres downwind of mountains. Lee waves can cause strong gusts at ground level and sometimes severe turbulence high in the air which can be a major hazard for aviation. One benefit of ENDGame is a much better representation of these waves which can now be reliably forecast using the UKV model.

Enhancing physical parametrizations

One of the major improvements to the physical parametrizations in the regional models is the 'blended turbulence scheme' which improves the way the mixing generated by turbulent eddies is represented in the model.

Dr Adrian Lock, Met Office Science Fellow, describes the new scheme:

"The changes mean that the scheme is able to adapt as these eddies start to become explicitly resolved on the model grid. Along with improvements in how satellite observations of cloud are incorporated into the UKV analysis (our best estimate of the current state of the atmosphere) this leads to a better representation of stratocumulus cloud (sheets of low cloud which typically occur in high pressure systems). These clouds are important to get right, especially for near-surface temperature forecasts in the winter, as they restrict cooling of the surface at night, reducing the chance of fog or icing, for example."

While these model changes are beneficial, there is still much room for improvement. Over the coming years, enhancements to our supercomputer will enable us to explore the potential benefits to be gained from increasing the resolution of our regional models, including testing city-scale models with grid-spacings as fine as a few hundred metres.

However, to realise the full benefits of higher resolution it is vital that we make further improvements to the parametrization schemes. We will work with our UK and international partners to achieve this, challenging the models with networks of detailed observations so we can improve our weather and climate services into the next decade and beyond.